Remove the saucepan from the heat. While the electric mixer is running, pour the syrup in a slow, steady stream down the side of the mixing bowl into the egg-yolk mixture (it is essential that the syrup touches the side of the bowl as you pour it in so the sugar will be very evenly incorporated and not splatter onto the sides of the bowl) until the syrup has been completely incorporated, about 1 1/2 minutes. Beat until the bowl is just slightly warm to the touch, 5 to 6 minutes.

Add butter, one piece at a time, until all of it has been completely incorporated and the frosting is fluffy, about 4 minutes more. Use immediately.

Cook's Notes

This frosting can be refrigerated in an airtight container for up to 2 days. Before using, bring to room temperature and beat with a hand whisk or the whisk attachment of an electric mixer for several minutes to restore the fluffiness.